1) Field of the Invention
The present invention relates to a process for preparing 3,4-dihydroxybenzonitrile.
2) Description of the Related Art
3,4-dihydroxybenzonitrile is an important starting material for the preparation of a number of valuable chemical compounds, including thiazole, 2-oxazolines, tetraazoles, imidazole, triazoles, and benzamidines. Several conventional processes for preparing 3,4-dihydroxybenzonitrile have been known for a long time. For example, in Tetrahedron Letters 1978, 52, 5183-5186, McCarthy et al. reported a process for preparing 3,4-dihydroxybenzonitrile by heating 3,4-dimethoxybenzonitrile in dimethyl formamide (DMF) in the presence of an excess amount of sodium cyanide at 180xc2x0 C. for 24 hours with a yield of 44%. Hwu et al. (J. Org. Chem. 1997, 62, 4097-4104) described a process for the preparation of 3,4-dihydroxybenzonitrile by heating 3,4-dimethoxybenzonitrile in a mixture of tetrahydrofuran (THF) and 1,2-dimethyl-2-imidazolidinone (DMEU) in the presence of either sodium bis(trimethylsilyl)amide [NaN(SiMe3)2] or lithium diisopropylamide [LiN(Pri)2] at 185xc2x0 C. for 12 hours in a sealed tube. In this case, an excess amount of lithium diisopropylamide [LiN(Pri)2] will be required for a higher yield of 3,4-dihydroxybenzonitrile. Accordingly, Hwu et al. (Synthesis 1998, 329-332) further reported a process for the preparation of 3,4-dihydroxybenzonitrile by heating methyl 3,4-dimethoxybenzoate in a mixture of tetrahydrofuran (THF) and 1,2-dimethyl-2-imidazolidinone (DMEU) in the presence of either sodium bis(trimethylsilyl)amide [NaN(SiMe3)2] or lithium diisopropylamide [LiN(Pri)2] at 185xc2x0 C. for 24 hours in a sealed tube with a yield of 63%.
Further, Feng and co-workers (Synthetic Communications 1998, 28(20):3765-3768) reported a process for preparing 3,4-dihydroxybenzonitrile by microwave irradiating 3,4-dihydroxybenaldehyde with hydroxyammonium salt in the presence of HCOOH/SiO2 with a yield of 70%. By using N-methylpyrrolidinone (NMP) under microwave irradiation, Chakraborti and co-worker (Tetrahedron 1999, 55, 13265-13268) reported that an improved yield of 89% can be achieved.
However, the aforementioned processes for preparing 3,4-dimethoxybenzonitrile are less than fully satisfactory in view of low yields of product, expensive starting material and solvents, and hazardous reagents, thereby limiting their use in commercial scale production.
Therefore, it is an object of this invention to provide a process for the preparation of 3,4-dihydroxybenzonitrile particularly characterized by high yield and a low cost starting material.
According to one aspect of this invention, the process for preparing 3,4-dihydroxybenzonitrile comprises the steps of reacting a nitrile compound of formula (I) 
wherein R1 and R2 are independently selected from the group consisting of H and C1-C4 alkyl with the proviso that R1 and R2 can not be H at the same time, or R1 and R2 may taken together to form xe2x80x94CH2xe2x80x94, with an alkali metal halide in the presence of a solvent and followed by treating with an acid to produce 3,4-dihydroxybenzonitrile.
According to another aspect of this invention, the process for preparing 3,4-dihydroxybenzonotrile comprises the steps of converting an aldehyde compound of formula (II) 
into a nitrile compound of formula (I) 
wherein R1 and R2 independently represent H or a C1-C4 alkyl group, with the proviso that R1 and R2 can not be H at the same time, or R1 and R2 may taken together to form xe2x80x94CH2xe2x80x94; reacting the nitrile compound with an alkali metal halide compound in the presence of a solvent; and followed by treating with an acid to produce 3,4-dihydroxybenzonitrile.
The process of this invention for preparing 3,4-dihydroxybenzonitrile includes the steps of: (a) converting an aldehyde compound of formula (II) 
into a nitrile compound of formula (I) 
wherein R1 and R2 independently represent H or a C1-C4 alkyl group, with the proviso that R1 and R2 can not be H at the same time, or R1 and R2 may taken together to form xe2x80x94CH2xe2x80x94; (b) reacting the nitrile compound obtained from step (a) with an alkali metal halide compound in the presence of a solvent; and (c) followed by treating with an acid to produce 3,4-dihydroxybenzonitrile.
Preferably, in formula (I), R1 and R2 are independently selected from a group consisting of methyl and ethyl.
In the preferred embodiments, the alkali metal halide compound is selected from a group consisting of lithium bromide, lithium chloride, sodium iodide and potassium iodide. Preferably, the alkali metal halide compound is lithium bromide.
Preferably, suitable solvent for use in the process of the present invention is dimethyl formamide (DMF).
Preferably, reaction of the nitrile compound of formula (I) with the alkali metal halide compound is carried out at a temperature in a range of 160-180xc2x0 C. for 10 hours.